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The VisionX file system has the capability to store sets
of elements in a single "packed" element. The
advantage of this capability is file i/o performance. A
large number of elements may be read with a single system
read. By default, any packed element will be unpacked by the
read functions VXread and VXreadframe which makes packing
transparent to the user (or programmer).
Data packing is especially useful for graphics data files
that involve a very large number of small elements.
VXpack and VXupack allow the global strategies
for packing and unpacking to be set.
VXupack(0) sets the default mode of completely unpacking
all packed elements. VXupack(1) sets a fast strategy that
does not perform mallocs for each unpacked element. The down
side of this capability is that memory assigned to elements
can no longer be deallocated when an element is deleted.
This mode offers performance enhancement for may graphics
applications but will likely cause memory problems for
applications involving very large files. Once this mode has
been set, memory deallocation cannot be reestablished by
setting VXupack to a different mode. VXupack(2) inhibits
automatic unpacking of packed elements (packed elements may
be explicitly unpacked with VXupklist()).
VXpack(0) diasables automatic packing which is the
default. VXpack(1) sets automatic packing for the functions
VXwrite() and VXwriteframe(). In this mode, the whole list
will be packed into a single packed element. Frame start and
frame end elements are not packed so that frame i/o can
still be performed; however, the elements between frame
elements wil be packed. Note, after a call to VXwrite or
VXwriteframe in this mode the list passed to these functions
will be in packed form as if VXpklist() had been called on
it.
The functions VXupklist() and VXpklist()
permit the explicit packing and unpacking of lists.
VXpklist() will pack a list according to the rules specified
for VXpack(1). VXupklist() will unpack a packed element in a
list.
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